Adhesives, particularly conductive adhesives, are used for a variety of purposes in the fabrication and assembly of semiconductor packages and microelectronic devices. The more prominent uses include bonding of electronic elements such as integrated circuit chips to lead frames or other substrates, and bonding of circuit packages or assemblies to printed wire boards. Adhesives useful for electronic packaging applications typically exhibit properties such as good mechanical strength, curing properties that do not affect the component or the carrier, and thixotropic properties compatible with application to microelectronic and semiconductor components. In addition, such adhesives must be able to withstand local temperature fluctuations present during use of e.g., a circuit board without losing strength or adhesiveness, and should not be hydrophilic because absorption of atmospheric moisture can also degrade strength and adhesiveness during thermal cycling.
Thermoset compounds that are useful for microelectronic adhesives include (meth)acrylate monomers. These compounds are typically polymerized by means of free radical initiators. This is a rapid reaction and this type of cure is well suited for high-throughput industrial processes.
Monomer backbones containing ether linkages are attractive because of their superior hydrolytic resistance. However, only a limited range of (meth)acrylate monomers with polyether backbones are currently available. Commercially available polyether-based (meth)acrylate monomers fall into three general categories: derivatives of polyethylene glycol, derivatives of polypropylene glycol, and ethylene oxide adducts of bisphenol A. The polyethylene glycol derivatives are much too hydrophilic to be used in microelectronic applications. The polypropylene oxide derivatives are somewhat less hydrophilic, but have poor thermal stability. Some of the bisphenol A derivatives have properties that allow their use in microelectronic adhesives, but others do not. For example, the short chain adducts (i.e. around two moles ethylene oxide (EO) per bisphenol A) have high viscosities (i.e. over 1,000 centipoises), while the long chain adducts (equal to or greater than four moles EO per bisphenol A) are too hydrophilic for most microelectronic applications.
There remains a need therefore for hydrophobic, low viscosity, polyether based monomers that can fill a properties gap that is not met by the materials currently available in the marketplace.